This invention relates to digital printing, which includes both electrophotographic printing and photofinishing, and more particularly to a method of optimizing the print process for certain kinds of spatial light modulators, thereby improving print quality.
Spatial light modulators (SLMs) have found application in many fields, a significant one of which is digital printing. In general, an SLM is an array of light-emitting, light-transmitting, or light-reflecting elements, which are individually addressable, usually with electronic signals. Many SLMs are binary, having an addressing scheme that switches its elements to either an xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state to form the image. A characteristic of SLMs is that there is no scanningxe2x80x94all pixels are activated at substantially the same time to generate the entire image or a two-dimensional block of the image, depending on the size of the image and the SLM.
One type of SLM is a digital micro-mirror device (DMD). The DMD has an array of hundreds or thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts and each is spaced by means of an air gap over underlying addressing circuitry. The addressing circuitry provides electrostatic forces, which cause each mirror to selectively tilt.
For printing applications, the DMD is addressed with exposure data, and in accordance with the data, light is selectively reflected or not reflected from each mirror to a photosensitive surface. In the case of electrophotographic printing, the photosensitive surface is an OPC (organic photoconductive drum) or other photoreceptor, which then transfers a latent image to paper or other printable media. In the case of photofinishing, the photosensitive surface is the photosensitive paper that will bear a printed photograph. It should be noted that DMDs may also be successfully used for the exposure phase of variations of electrophotographic printing, i.e., electrophoretic printing.
For all types of digital printing, the DMD has proven itself to perform well in terms of print quality. Depending on the application, DMD characteristics and operation may be optimized according to consumer expectations of how the output should best appear and to industry demands. For example, for photofinishing applications, the resolution must be sufficiently high to compete with conventional analog photofinishing, yet the process must also be sufficiently efficient to make use of the DMD a cost effective alternative. Parameters such as mirror size and modes of modulation are design choices that can be varied according to the particular application. Providing the best design for a particular application requires an accurate model of the output characteristics of the DMD.
One aspect of the invention is an method of using a spatial light modulator for the exposure phase of digital printing. The spatial light modulator is of a type modeled by a steep-sided intensity/displacement xe2x80x9ccurvexe2x80x9d, which represents desirable features of the SLM. Also, the spatial light modulator has at least one hole in the center of each pixel element. The characteristics of the hole(s) are adjusted for optimum print quality, such as by adjusting the number of holes, the location- of the hole,(s), or the size of the hole(s). The characteristics of the hole determine further characteristics of the intensity/displacement curve, namely, the size and shape of a xe2x80x9cdipxe2x80x9d in the top of the curve. Optics can be used to process the light out of the spatial light modulator, and to thereby retain or flatten this xe2x80x9cdipxe2x80x9d so as to achieve a desired print quality.